Methods and apparatus for controlling plasma in a plasma processing system

ABSTRACT

Methods and apparatus for processing a substrate in a multi-frequency plasma processing chamber are disclosed. The base RF signal pulses between a high power level and a low power level. Each of the non-base RF generators, responsive to a control signal, proactively switches between a first predefined power level and a second predefined power level as the base RF signal pulses. Alternatively or additionally, each of the non-base RF generators, responsive to a control signal, proactively switches between a first predefined RF frequency and a second predefined RF frequency as the base RF signal pulses. Techniques are disclosed for ascertaining in advance of production time the first and second predefined power levels and/or the first and second predefined RF frequencies for the non-base RF signals.

CLAIM OF PRIORITY

This application is a divisional of and claims the benefit, under 35U.S.C. § 120, of U.S. patent application Ser. No. 14/792,527, filed onJul. 6, 2015, and titled “METHODS AND APPARATUS FOR CONTROLLING PLASMAIN A PLASMA PROCESSING SYSTEM”, which is a continuation of and claimsthe benefit, under 35 U.S.C. § 120, of U.S. patent application Ser. No.13/531,491, filed on Jun. 22, 2012, and titled “METHODS AND APPARATUSFOR CONTROLLING PLASMA IN A PLASMA PROCESSING SYSTEM”, now issued asU.S. Pat. No. 9,114,666, which claims the benefit of and priority, under35 U.S.C. § 119(e), to U.S. Provisional Patent Application No.61/602,040, filed on Feb. 22, 2012, and titled “FREQUENCY ENHANCEDIMPEDANCE DEPENDENT POWER CONTROL FOR MULTI-FREQUENCY RF PULSING”, andto U.S. Provisional Patent Application No. 61/602,041, filed on Feb. 22,2012, and titled “METHODS AND APPARATUS FOR SYNCHRONIZING RF PULSES IN APLASMA PROCESSING SYSTEM”, all of which are incorporated by referenceherein in their entirety.

BACKGROUND

Plasma processing has long been employed to process substrates (e.g.,wafer or flat panels or other substrates) to create electronic devices(e.g., integrated circuits or flat panel displays). In plasmaprocessing, a substrate is disposed in a plasma processing chamber,which employs one or more electrodes to excite a source gas (which maybe an etchant source gas or a deposition source gas) to form a plasmafor processing the substrate. The electrode may be excited by an RFsignal, which is furnished by an RF generator, for example.

In some plasma processing systems, multiple RF signals, some of whichmay have the same or different RF frequencies, may be provided to thesubstrate-bearing electrode (also referred to herein as the lowerelectrode or chuck) to generate plasma while the upper electrode isgrounded. In a capacitively-coupled plasma processing system, forexample, one or more RF signals may be provided to the bottom electrodewhile the top electrode is grounded.

In some applications, the plurality of RF signals may be pulsed. For anygiven RF signal, RF pulsing involves turning the RF signal on and off(or alternating between a high power level and a low power level sincepulsing does not always require the power to be turned off) at a pulsingfrequency that may be different from (and typically slower than) the RFfrequency. Generally speaking, RF pulsing is performed in the past toimprove certain processing results (such as to improve uniformity orreduce etching-related damage).

The pulsing of the various RF signals may be unsynchronized orsynchronized. With respect to synchronized pulsing, for example, if twosignals RF1 and RF2 are synchronized, there is an active pulse of signalRF1 for every active pulse of signal RF2. The pulses of the two RFsignals may be in phase, or the leading edge of one RF pulse may lagbehind the leading edge of the other RF pulse, or the trailing edge ofone RF pulse may lag behind the trailing edge of the other RF pulse, orthe RF pulses may be out of phase.

If the pulsing of the various RF signals is not well-controlled, thereis a risk that RF power instability resulting in plasma perturbation mayoccur during the transition from low to high (or vice versa) of one ormore of the RF signals. This is because during such a transition by oneor more of the RF signals, the plasma condition in the processingchamber changes. Such change may be detected by the match network and/orthe other RF generators, which may attempt to compensate for thedetected plasma condition changes. The reactive nature of suchcompensation means that for the duration between a plasma conditionchange detection and successful compensation, RF power perturbationsresulting in plasma instability exist.

FIG. 1 shows an example of such RF power perturbation, which may resultin plasma instability during the transition of one of the pulsing RFsignals. In the example of FIG. 1 , the 2 MHz RF signal pulses at 100 Hzwith a 50% duty cycle between 2,500 W and 0 W. For illustrationpurposes, suppose the 60 MHz RF signal operates in the continuouswaveform (CW) mode without pulsing. As the 2 MHz RF signal transitionsfrom the low state 102 to the high state 104, the plasma conditionwithin the chamber changes in response to the changing power supplied.The 60 MHz RF signal, upon detecting such plasma condition change, isshown compensating (either via the compensation circuit in the 60 MHz RFpower supply or in the match network) for the detected plasma conditionchange.

However, this is a reactive response and depends on first detecting theplasma condition change brought about by the low-to-high transition ofthe 2 MHz pulsing RF signal (which pulses at a pulsing frequency of 100Hz as mentioned earlier). The delay and subsequent response causes theRF power level perturbation shown by reference number 106, which shows atemporary dip in the power level of the 60 MHz RF signal after the 2 MHztransitions from low to high. Another instance of RF power levelperturbation in the 60 MHz RF signal due to the delayed response of the60 MHz RF signal is shown by reference number 108 after the 2 MHz RFtransitions from high (110) to low (112). Other RF power perturbationsare shown by reference numbers 114 and 116 in FIG. 1 , for example. Ascan be seen in FIG. 1 , these RF power perturbations may be in thepositive direction or negative direction and may have differentintensities. Such perturbations result in unstable and/or poorlycontrolled plasma events, affecting process results and/or device yield.

Furthermore, modern plasma processes impose stringent process resultrequirements in the fabrication of high density, high performancedevices. Some process windows cannot be reached or are quite narrow withtraditional constant waveform RF signals or with traditional RF pulsingmethods.

Manipulating and further control of the pulsing of various RF signals toimprove plasma stability and/or to provide additional process controlknobs are among the many goals of embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 shows an example of such RF power perturbation, which may resultin plasma instability during the transition of one of the pulsing RFsignals.

FIG. 2 shows, in accordance with an embodiment of the invention, asimplified capacitively coupled plasma processing system having a plasmaprocessing chamber and configured for power level control of the variousRF signal pulsing states.

FIG. 3 shows a plot of delivered power versus time for two RF signals toillustrate the effect of proactively synchronizing the pulsing among thevarious RF signals.

FIG. 4 shows, in accordance with an embodiment of the invention, thesituation wherein the 60 MHz RF signal has its power levels adapted tothe pulsing states of the 2 MHz RF signal.

FIG. 5 shows, in accordance with another embodiment of the invention,the situation wherein the 60 MHz RF signal has its power levels adaptedto the pulsing states of the 2 MHz RF signal.

FIG. 6 shows, in accordance with another embodiment of the invention,the situation wherein the 27 MHz RF signal and the 60 MHz RF signal havetheir power levels adapted to the pulsing states of the 2 MHz RF signal.

FIG. 7 shows a proof of concept plot, illustrating the fact that undercertain conditions, the non-base RF generator is unable to deliver RFpower at the required power set point.

FIG. 8 shows, in accordance with an embodiment of the invention, amethod for learning the optimal tuned RF frequencies for the non-base RFgenerator while the base RF generator pulses.

FIG. 9 shows, in accordance with an embodiment of the invention, amethod for delivering optimal RF power to the plasma load in a plasmachamber while the plasma chamber is provided with a pulsing base RFsignal and at least one non-base RF signal.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference toa few embodiments thereof as illustrated in the accompanying drawings.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

Various embodiments are described herein below, including methods andtechniques. It should be kept in mind that the invention might alsocover articles of manufacture that includes a computer readable mediumon which computer-readable instructions for carrying out embodiments ofthe inventive technique are stored. The computer readable medium mayinclude, for example, semiconductor, magnetic, opto-magnetic, optical,or other forms of computer readable medium for storing computer readablecode. Further, the invention may also cover apparatuses for practicingembodiments of the invention. Such apparatus may include circuits,dedicated and/or programmable, to carry out tasks pertaining toembodiments of the invention. Examples of such apparatus include ageneral-purpose computer and/or a dedicated computing device whenappropriately programmed and may include a combination of acomputer/computing device and dedicated/programmable circuits adaptedfor the various tasks pertaining to embodiments of the invention.

Embodiments of the invention relate to methods and apparatus forcontrolling plasma processes by proactively setting the RF power levelsof one or more higher frequency RF signals and proactively control thepulsing to minimize RF power perturbation during processing. The powerlevels of the higher frequency RF signals are determined and then setseparately responsive to the pulsing state of the base pulsing RFsignal. In other words, the power levels of the higher frequency RFsignals are determined and then set separately for the high pulse of thebase pulsing RF signal and for the low pulse of the base pulsing RFsignal.

As the term is employed herein, the base pulsing RF signal representsthe lowest frequency RF signal that pulses. For example, if the lowerelectrode is provided with three RF signals (2 MHz, 27 MHz, and 60 MHz)and the 2 MHz RF signal pulses, the 2 MHz RF signal represents the basepulsing RF signal since it is the lowest frequency RF signal thatpulses. As another example, if a plasma processing chamber is providedwith three RF signals to its lower electrode (2 MHz, 27 MHz, and 60 MHz)and the 2 MHz RF signal is operated in the continuous waveform (i.e.,non pulsing mode) and the 27 MHz RF signal and the 60 MHz RF signalpulse, the 27 MHz RF signal represents the base pulsing RF signal.

To clarify terms, the base pulsing RF signal may be the same or may bedifferent from a master RF signal, which represents the independentlypulsing RF signal. When multiple RF power supplies pulse, one of the RFpower supplies may be designated the master RF power supply and pulsesits master RF signal independently. The master RF power supply may issuecontrol signals to other RF power supplies to synchronize pulsing. Thereis no requirement that the master RF signal be the lowest frequency RFsignal. Thus, the 27 MHz pulsing RF signal may act as a master for the 2MHz pulsing RF signal, and vice versa. However, the base pulsing RFsignal is, as the term is employed herein, the lowest frequency RFsignal that pulses. It should be noted at this point that the use of amaster RF power supply to synchronize pulsing among RF power supplies isonly one way to synchronize pulsing. An external circuit may be employedto synchronize the pulsing among all RF power supplies, for example.

In one or more embodiments, as the base RF signal pulses, each of theother pulsing RF signals proactively alternates between its first presetpower level and its second preset power level in synchronization withthe pulsing states of the base pulsing RF signal. The first preset powerlevel represents the other pulsing RF signal's power level that isestablished for the high pulse of the base RF signal. The second presetpower level represents the other pulsing RF signal's power level that isestablished for the low pulse of the base RF signal.

For example, suppose a plasma processing chamber is provided with threeRF signals to its lower electrode (2 MHz, 27 MHz, and 60 MHz) and boththe 2 MHz and the 27 MHz RF signals pulse at 100 Hz. The 2 MHz basepulsing RF signal would pulse between a 2 MHz high power level and a 2MHz low power level at 100 Hz. The 27 MHz RF signal would proactively,responsive to a control signal from a master RF power supply or anexternal synchronization control circuit, alternate between a firstpreset power level (which occurs responsive to the 2 MHz high powerlevel) and the second preset power level (which occurs responsive to the2 MHz low power level).

The preset power levels of the non-base pulsing RF signal (such as the27 MHz RF signal in the previous example) are ascertained and/orestablished to achieve certain desired processing results. Further, eachof the first preset power level and the second preset power level of thenon-base pulsing RF signal is established independently for each pulsingstate of the base pulsing RF signal. As such, they are independentlyascertained and/or established for the plasma conditions that existduring the high state of the base RF signal (such as the 2 MHz RF signalin the previous example) and the low state of the base RF signal. Oncethese preset power levels are established for the non-base RF signal(e.g., during recipe formation), the non-base RF signal wouldproactively, responsive to a control signal from the master RF powersupply or from an external synchronization control circuit, alternatebetween the first preset power level and the second preset power levelduring production (e.g., during substrate processing) as the basepulsing RF signal pulses between its high state and its low state.Another way of stating this is that the non-base pulsing RF signalresponse depends not only to the fact that the base RF signal pulses butalso to the state (high or low) of the base RF signal.

In one or more embodiments, proactive response is used to optimize RFpower instability during pulsing. As the term is employed herein,proactive or proactive response refers to the fact that compensationand/or pulsing of the RF signals is performed proactively instead ofreactively. As discussed earlier, reactive response occurs when thematch network or when the RF power supply associated with an RF signaldetects that the plasma condition (such as plasma impedance, forexample) has changed in the chamber due to the pulsing of one of theother RF signals. In the reactive response mode, after such detectionoccurs, the match network or the RF power supply responds to compensatefor the detected plasma condition change. To elaborate, in the reactiveresponse mode, the match network or the RF power supply responds onlyafter detection is made.

In contrast, in the proactive response mode, the response of the matchnetwork or the RF power supply of the other RF signal is proactivelyinitiated by a control signal without having to wait for the detection.For example, an external control circuit and/or processor and/orcomputer may proactively send a control signal to instruct the matchnetwork or an RF power supply to respond based on its knowledge of thepulsing behavior/timing of one or more of the other RF signals. Suchcontrol signal and response occur without having to wait for thedetection of the pulse-related plasma condition change to occur. Asanother example, the RF generator for one of the pulsing RF signals maycommunicate with other RF generators to provide control signals toinitiate the responses by the other RF generators. In this case, the RFgenerator that issues the control signal would act as the master RFgenerator, and other RF generators act as slave RF generators. Themaster RF generator issues control signals to its slave RF generatorsproactively instead of in response to the detection of plasma conditionchange.

By proactively controlling the response of the match network and/or theresponse of other RF generators, the RF power instability and/or theplasma perturbation due to pulsing is reduced in duration and/or inintensity. In this manner, power perturbation is reduced and plasmastability is enhanced.

The features and advantages of embodiments of the invention may bebetter understood with reference to the figures and discussions thatfollow.

FIG. 2 shows, in accordance with an embodiment of the invention, asimplified capacitively coupled plasma processing system 202 having aplasma processing chamber 204. Although a typical plasma processingsystem may have multiple chambers, only one chamber is shown forillustration purpose. Also omitted are other details well-known to thoseskilled in the art such as robot transfer arms, storage cassettes, gassupplies, etc.

In the example of FIG. 2 , upper electrode 206 is grounded while lowerelectrode 208, representing the substrate holder or chuck, is suppliedwith three RF signals (2 MHz, 27 MHz, and 60 MHz) from three RF powersupplies 220, 222, and 224 respectively through matching network 230.Although three RF signals and three RF power supplies are shown, lowerelectrode 208 may be supplied with as few as one or as many RF signalsas desired. Further, although the RF frequencies of 2 MHz, 27 MHz, and60 MHz are selected for illustration, different RF frequencies may beused if desired. As illustrated, plasma processing chamber 204 isconfigured for dielectric etching.

As is known by those skilled in the art, matching network 230 matchesthe impedance of the RF power sources 220, 222, and 224 with theimpedance of the plasma load in plasma processing chamber to minimizethe reflected power and to maximize power delivery. In accordance withan embodiment of the invention, the RF power sources 220, 222, and 224are in communication such that if one of the RF power sources acts as anRF pulse master, that RF power source can send control signalsproactively to other RF signals in order to proactively initiate thepulsing of these other RF signals.

For example, RF power source 220 (the 2 MHz RF power source) may act asthe pulse master and send digital or analog control signals (which maybe electrical or optical signals, for example) via conduits 230 and 232to RF power sources 222 and 224 respectively to instruct RF powersources 222 and 224 to synchronize their pulsings with the master 2 MHzRF signal pulse (for example with the leading edge, the trailing edge,or any predetermined time in the 2 MHz RF pulse period) without havingto wait for a detection of the plasma change condition in plasmaprocessing chamber 204.

As another example, RF power source 222 (27 MHz RF power source) may actas the pulse master and send digital or analog control signals (whichmay be an electrical or an optical signal, for example) via conduits 234and 232 to RF power sources 220 and 224 respectively to instruct RFpower sources 220 and 224 to synchronize their pulsings with the master27 MHz RF signal pulse (for example with the leading edge, the trailingedge, or any predetermined time in the MHz RF signal pulse period)without having to wait for a detection of the plasma change condition inplasma processing chamber 204.

Alternatively, a control circuit 250 may be used provide control signalsto all three RF power sources 220, 222, and 224 as shown. In this case,none of the RF power sources would need to act as a master and all mayreceive control signals that proactively instruct the RF power sourcesto pulse from control circuit 250. By proactively controlling thepulsing of the various RF signals, RF power perturbation is minimized aswill be shown in FIG. 3 below.

FIG. 3 shows a plot of delivered power versus time for two RF signals: a2 MHz RF signal 302 and a 60 MHz RF signal 304. The 2 MHz RF signal 302pulses at 100 Hz with 50% duty cycle between 2,500 W and 0 W. Forillustration clarity, the 60 MHz RF signal 304 pulses with its firstpreset power level set at 900 W for the high pulse duration of the 2 MHzRF signal and its second preset power level also set at 900 W for thelow pulse duration of the 2 MHz RF signal in the example of FIG. 3 . Inthe example of FIG. 3 , both the 2 MHz and the 60 MHz RF power sourcesthat generate these pulsed RF signals receive a control signal from acommon control circuit (such as control circuit 250 of FIG. 2 ) and thuspulse proactively in a simultaneous manner without waiting for adetection of the plasma condition change. Alternatively, one of the RFpower sources (e.g., either the 2 MHz RF power source or the 27 MHz RFpower source) may act as a pulse master for the other RF power sourceand may issue a control signal proactively to instruct the other RFpower source to pulse substantially simultaneously without waiting for adetection of a plasma condition change.

As can be seen in FIG. 3 , power perturbation in the 60 MHz RF signal304 is kept substantially lower (below 3% in the case of FIG. 3 ) foreach high-to-low or low-to-high transition of the 2 MHz base pulsing RFsignal 302. These are shown by reference numbers 320, 322, 324, and 328.There is substantially less RF power perturbation compared to thesituation in FIG. 1 wherein the 60 MHz RF power source operates in thereactive mode (i.e., compensates responsive to a detection of a plasmacondition change). This is because once the power level set points aredetermined for the non-base pulsing RF signals, the non-base pulsing RFsignals can pulse between its two power set points without delay uponreceiving a control signal, thereby contributing to a more stable RFpower delivery.

In one or more embodiments, each of the first power level and the secondpower level of the non-base pulsing RF signals (i.e., higher RFfrequency pulsing signals) may be dynamically ascertained from one ormore measurable plasma processing chamber parameters (such as chuck biasor return RF current, for example). By “dynamic”, it should beunderstood that this determination of the power levels may be donealgorithmically during recipe formation or machine calibration, forexample, or may be done on-the-fly during processing. The non-basepulsing RF signal's first power level (corresponding to the power levelof the non-base pulsing RF signal while the base pulsing RF signal is inits high state) may be algorithmically ascertained automatically fromsensor measurements of one or more plasma processing chamber parametersand computed for the high state of the base pulsing RF signal. Likewise,the non-base pulsing RF signal's second power level (corresponding tothe power level of the non-base pulsing RF signal while the base pulsingRF signal is in its low state) may be algorithmically ascertainedautomatically from sensor measurements of one or more plasma processingchamber parameters and computed for the low state of the base pulsing RFsignal.

In this case, the ability to dynamically determine and set the powerlevels of the non-base pulsing RF signals (i.e., the higher frequencypulsing RF signals), using a programmed computer for example, based onsensor measurements of one or more plasma processing chamber parametersduring the high state and low state of the base RF pulsing signal inorder to achieve a desired process parameter, represents an advantagesince the power levels of the higher frequency pulsing RF power signalsduring the high state of the base pulsing RF signal and during the lowstate of the base pulsing RF signal are now separate control knobs forthe process. Thereafter, the non-base pulsing RF signal simplytransitions from the first predefined power level to the secondpredefined power level (and vice versa) upon receiving control signalsfrom either the master RF power source or from an external controller(e.g., controller circuit 250 of FIG. 2 ).

Note that the first predefined power level and the second predefinedpower level are specific to each RF power supply. In other words, a 27MHz power supply may have its own first predefinedRF-power-supply-specific power level and second predefinedRF-power-supply-specific power level while a 60 MHz RF power supply mayhave its own first predefined RF-power-supply-specific power level andsecond predefined RF-power-supply-specific power level that aredifferent than those of the 27 MHz RF power supply.

In one or more embodiments, the power levels (i.e., the high pulsingpower level and the low pulsing power level) of the base pulsing RFsignal may also be determined dynamically, using a programmed computerfor example, based on sensor measurements of one or more plasmaprocessing chamber parameters (such as chuck bias) in order to achieve adesired process parameter (such as deposition rate). The ability todynamically determine the power levels of the base pulsing RF signal,using a programmed computer for example, based on sensor measurements ofone or more plasma processing chamber parameters in order to achieve adesired process parameter represents an advantage since the power levelsof the base pulsing RF power signals are now control knobs for theprocess.

FIG. 4 shows, in accordance with an embodiment of the invention, thesituation wherein the 2 MHz RF signal 402 is pulsed at 100 Hz with 50%duty cycle between 2,500 W and 0 W. The 60 MHz RF signal 404 issynchronized in a proactive manner such that for the high state of the 2MHz RF signal, the power level of the 60 MHz RF signal is at 900 W andduring the low state of the 2 MHz RF signal, the power level of the 60MHz RF signal is 450 W. Note that each of these two power levels of the60 MHz RF signal is separately determined and set in view of thespecific state (high or low) of the base RF pulsing signal (e.g., the 2MHz RF signal) and is determined and set to achieve a desired processresult (such as low polymer deposition, reduced wafer bias, etc.). Also,pulsing of the two RF signals of FIG. 4 occurs proactively, i.e.,without waiting on the detection of a change in plasma condition or adetection of changes in one or more chamber parameters that reflect suchplasma condition change due to base RF signal pulsing. As such, RF powerperturbation and RF plasma instability are greatly reduced.

FIG. 5 shows, in accordance with another embodiment of the invention,the situation wherein the 2 MHz RF signal 502 is pulsed at 100 Hz with50% duty cycle between 2,500 W and 0 W. The 60 MHz RF signal 504 issynchronized in a proactive manner such that for the high state of the 2MHz RF signal, the power level of the 60 MHz RF signal is at 900 W.During the low state of the 2 MHz RF signal, the power level of the 60MHz RF signal is increased to 1,125 W. FIG. 4 and FIG. 5 illustrate thatthe power level of the non-base RF signal (i.e., the higher frequencypulsing RF signal) may be higher or lower after the base pulsing RFsignal transitions from a high state to a low state. Again, note thateach of these two power levels of the 60 MHz RF signal is determined andset in view of the specific state (high or low) of the base RF pulsingsignal (e.g., the 2 MHz RF signal) and is determined and set to achievea desired process result (such as low polymer deposition, reduced waferbias, etc.). Also, pulsing of the two RF signals of FIG. 4 occursproactively, i.e., without waiting on the detection of a change inplasma condition. As such, RF power perturbation and RF plasmainstability are greatly reduced.

FIG. 6 shows, in accordance with another embodiment of the invention,the situation wherein the 2 MHz RF signal 602 is pulsed at 100 Hz with50% duty cycle between 2,500 W and 0 W. The 60 MHz RF signal 604 ssynchronized in a proactive manner such that for the high state of the 2MHz RF signal, the power level of the 60 MHz RF signal is at 500 W.During the low state of the 2 MHz RF signal, the power level of the 60MHz RF signal is increased to 625 W. The 27 MHz RF signal 606 issynchronized in a proactive manner such that for the high state of the 2MHz RF signal, the power level of the 27 MHz RF signal is at 1,000 W.During the low state of the 2 MHz RF signal, the power level of the 27MHz RF signal is decreased to 250 W. Again, note that each of these twopower levels of the 60 MHz RF signal and of the 27 MHz RF signal isdetermined and set in view of the specific state (high or low) of thebase RF pulsing signal (e.g., the 2 MHz RF signal) and is determined andset to achieve a desired process result (such as low polymer deposition,reduced wafer bias, etc.).

In the case of FIG. 6 , one of the RF power sources (such as either the2 MHz RF power source, the 27 MHz RF power source, or the 60 MHz RFpower source) may act as the master and may send control signals to theother RF power sources to synchronize pulsing proactively.Alternatively, an external control circuit can act as a master and maysend control signals to all three RF power sources to synchronizepulsing proactively.

As mentioned, the RF power levels of the various RF signals may be tunedseparately during the high state of the base pulsing RF signal andduring the low state of the base pulsing RF signal to achieve a desiredprocess result. As an example, it is believed that ion energy may beincreased by increasing the RF power level of the 2 MHz RF signal.Increasing ion energy is beneficial may also result in unwantedexcessive polymer deposition in some cases. Pulsing the 2 MHz RF signalat its optimum pulsing frequency and optimum high and low RF powerlevels may result in increased ion energy without suffering from unduepolymer deposition.

Once the optimum RF power levels of the base pulsing RF signal aredetermined and/or set, the RF power levels of the non-base pulsing RFsignals (i.e., the higher frequency pulsing RF signals) may beseparately determined and set for the high state of the base pulsing RFsignal and the low state of the base pulsing RF signal to further tunethe process (such as to tune the plasma density since the power levelsof the higher frequency RF signals tend to affect plasma density more).As such, these different RF power levels of different RF pulsing signalsmay be employed as separate control knobs for the process.

As mentioned, since the various pulsing RF signals are proactivelysynchronized, RF power perturbations are minimized. Even if an RF signal(such as the 60 MHz RF signal) is specified by the recipe to operate ina continuous waveform (CW) mode, it may be desirable, in one embodiment,to set such RF signal to proactively pulse at the same power level (suchas 900 W) during both the high state of the base pulsing RF signal (suchas the 2 MHz RF signal) since as shown in FIG. 3 , such pulsing at thesame power level reduces RF power perturbation compared to the reactivecompensation approach.

In one or more embodiments, the RF power levels of the base pulsing RFsignal and the non-base RF signals are set such that plasma is sustainedduring pulsing. In other words, the plasma is not extinguished duringthe low state of either the base pulsing RF signal and/or the low stateof the non-base pulsing RF signal. Keeping the plasma ignited allowsprocess control to be more efficiently exerted by the process knobs ofseparate RF power levels (as mentioned earlier) and also minimizesplasma disturbance since plasma striking and/or ignition (which wouldhave been necessary if the plasma is allowed to extinguish) is not aswell-controlled a process as a continuous plasma. As such, repeatabilityand uniformity is enhanced.

In one or more embodiments, bimodal auto-frequency tuning techniques andapparatus therefor are disclosed. In a bimodal auto-frequency tuningapproach, the tuned frequency of the non-base RF signal is proactivelychanged as the base RF signal pulses from one state to another state toensure efficient and stable power delivery for every state of the baseRF signal.

To elaborate, modern RF power supplies are capable of tuning itsdelivered RF frequency to improve power delivery (e.g., by changing theRF frequency delivered to the load). As an example, a 60 MHz RFgenerator may be capable of changing its tuned RF frequency by, forexample, 5 to 10% (i.e., changing the RF frequency delivered to the loadby 60 MHz +/−5% to 10%).

However, such frequency change has been performed, so far, as anafter-the-fact response of the RF generator when its sensors detect achange in the amount of RF power delivered to the load. Such detectionoften depends on a measurement of, for example, the ratio of thereflected power to the forward power (also known as gamma). When the RFgenerator detects a condition that is characteristic of inefficientpower delivery (based on some preset gamma threshold, for example), theRF generator would, in a frequency tuning scheme, change its tuned RFfrequency in order to more efficiently deliver power to the load.

However, the after-the-fact nature of the current frequency tuningscheme often means that there is a lag in the response when changesoccur to the plasma impedance or the plasma load. During this lag time,under certain conditions such as when the base RF signal pulses from onestate to another state, the non-base RF generator may be highlyinefficient or may be unable to deliver power at the required power setpoints (as specified by the recipe) until the non-base RF generatorchanges its tuned frequency sufficiently to adapt to the changed plasmaload.

In accordance with one or more embodiments of the invention, the tunedfrequency of the non-base RF signals are ascertained in advance forevery pulse state (e.g., high or low) of the base RF signal. Considerthe situation, for example, when the 2 MHz base RF signal pulses atabout 1 kHz with a 50% duty cycle. For example, during the learningphase, it may be ascertained that when the 2 MHz base RF signal is inits low pulse state (i.e., after pulsing low), power delivery by the 60MHz RF generator is efficient when the 60 MHz RF generator actuallydelivers its RF power using a 61 MHz tuned frequency. Further, it may beascertained, in another example, in the learning phase that when the 2MHz RF signal is in its high pulse state (e.g., after pulsing high), the60 MHz RF generator is efficient when the 60 MHz RF generator actuallydelivers its RF power using a 59 MHz tuned frequency.

In one or more embodiments, during production, the 60 MHz RF generator(i.e., the non-base RF generator in this example) would proactivelychange its tuned frequency at the same time as the base RF signal pulsesfrom one state to another state. The frequency change is said to beproactive since the change in the tuned frequency by the non-base RFgenerator is not made based on an after-the-fact detection of a changein the plasma condition or a change in the impedance seen by the 60 MHzRF generator due to the pulsing of the base RF signal.

Rather, the change in the tuned RF frequency by the non-base RFgenerator is synchronized such that the change occurs at an optimal timeto ensure adequate and/or efficient power delivery as the base RF signalpulses from one state to another state. For example, the 60 MHz RFgenerator may change its RF tuned frequency proactively based on acoordination signal (which may be issued by the 2 MHz RF generator orany one of the RF generators or by a separate control circuit thatcoordinates the RF generators) instead of waiting for a detection of achange in the plasma condition or a change in the impedance seen by the60 MHz RF generator due to the pulsing of the base RF signal. Generallyspeaking, the non-base RF generator may change its tuned frequency atthe same time or even before the base RF signal pulses from one pulsingstate to another pulsing state.

FIG. 7 shows a proof of concept plot, illustrating the fact that undercertain conditions, the non-base RF generator is unable to deliver RFpower at the required power set point. This is the situation in theprior art, for example. In the example of FIG. 7 , the base 2 MHz RFgenerator has a power set point of 9 kW (not shown in FIG. 7 ), and the60 MHz RF generator has a power set point of 750 W. These are thedesired power levels for the respective RF generators. Further, the base2 MHz RF signal ramps for 5 seconds from a high state to a low state(from 2.2 seconds to 7.2 seconds) in the example of FIG. 7 .

In FIG. 7 , the left vertical axis represents the amount of powerdelivered by the 60 MHz RF generator while the right vertical axisrepresents the tuned frequency of the 60 MHz RF generator. Both verticalaxes are plotted against the horizontal time axis. Line 702 representsthe amount of RF power delivered. Line 730 represents the tunedfrequency of the 60 MHz RF generator.

At point 700, the 2 MHz RF generator is at the high pulse state. At thispoint, the 60 MHz RF generator delivers its power efficiently at a tunedRF frequency of approximately 61 MHz (line 730 at time t=2 seconds).

At time 2.2 seconds, the 2 MHz base RF signal begins to ramp low,reaching its low state at 7.2 seconds. As can be seen by the RF powerline 702, the 60 MHz RF generator senses the change in the plasma loadand attempts to maintain its power set point of 750 W. At some point intime, starting at about 5 seconds (point 704), the 60 MHz RF generatorbegins to change its tuned frequency downward in order to increase theRF power delivery efficiency in response to the detected change in theplasma load (which is caused by the 2 MHz base signal ramping to the lowstate).

At time 7.2 seconds (reference number 706 on the horizontal time axis),the 2 MHz base RF signal is at its low state. As can be seen in FIG. 7 ,the amount of RF power delivered by the 60 MHz RF generator temporarilydrops to about 220 W from point 708 to point 710. This amount of RFpower delivered by the 60 MHz RF generator is substantially below the750 W power set point for the 60 MHz RF generator. This represents anundesirable situation.

From point 706 to point 712, the 60 MHz RF generator hunts for the tunedRF frequency that would enable the delivery of 60 MHz RF power at therequired power set point of 750 W with the 2 MHz RF signal at its lowpulse state. At point 714, the 60 MHz RF generator settles to a tuned RFfrequency of about 59.75 MHz. At this lower tuned RF frequency, the 60MHz generator is able to deliver RF power at its set point of 750 Wagain.

The proof of concept plot of FIG. 7 illustrates that for every pulsestate of the 2 MHz base signal, there is an optimal tuned RF frequencyfor the 60 MHz RF generator. Further, if the 60 MHz generator changesits tuned RF frequency in an after-the-fact manner (i.e., afterdetecting a change in the plasma load due to the pulsing of the 2 MHzbase RF signal as per FIG. 7 ), there may exist situations where thepower set points and the RF frequencies are such that the 60 MHz RFgenerator would be unable to meet its required power set point. This isshown in FIG. 7 between points 706 and 712.

FIG. 8 shows, in accordance with an embodiment of the invention, amethod for learning the optimal tuned RF frequencies for the non-base RFgenerator while the base RF generator pulses. In step 802, the plasmachamber is powered with a pulsing base RF signal and at least onenon-base RF signal. In step 804, the non-base RF generator(s) is/areoperated in the auto-tuning mode to enable the non-base RF generator toseek its optimal RF frequencies (f1 and f2) for the high state and thelow state respectively of the base RF signal. In this auto-tuning mode,the non-base RF generator is allowed to seek its own tuned RF frequencyduring each of the states of the base RF signal. These optimal RFfrequencies (for the non-base RF signal) for each of the states of thebase RF signal serve as predefined RF frequencies, and the non-base RFgenerator proactively switches from one predefined RF frequency toanother predefined RF frequency as the base RF signal pulses.

As the term is employed herein, optimal RF frequency for the non-base RFgenerator refers to the RF frequency at which the non-base RF generatorcan deliver its power acceptably or efficiently (according to somepredefined criteria) and/or can meet its power delivery set point. Asdiscussed herein, there are at least two optimal RF frequencies for thenon-base generator. These two optimal RF frequencies correspond to thetwo switching states of the base RF signal.

Note that the first predefined RF frequency and the second predefined RFfrequency employed during production are specific to each RF powersupply. In other words, a 27 MHz power supply may have its own firstpredefined RF-power-supply-specific RF frequency and second predefinedRF-power-supply-specific RF frequency while a 60 MHz RF power supply mayhave its own first predefined RF-power-supply-specific RF frequency andsecond predefined RF-power-supply-specific RF frequency that aredifferent than those of the 27 MHz RF power supply.

In one or more embodiments, all other conditions of the chamber arepreferably arranged such that they mimic as closely as possible theconditions in production. In another embodiment, the frequency of thenon-base RF generator may be manually changed to ascertain (e.g., bymeasuring gamma) the optimal frequencies f1 and f2 for the high stateand the low state respectively of the base RF signal.

In step 806, these optimal non-base RF generator frequencies for thehigh state and low state of the base RF signal are recorded and/orstored for use during production (i.e., production of substrates afterthe optimal non-base RF generator frequencies were learned in thelearning phase). During production, the non-base RF generatorproactively switches between optimal RF frequency f1 and optimal RFfrequency f2 as the base RF signal pulses instead of waiting for thedetection of a change in the plasma impedance or in the gamma.

FIG. 9 shows, in accordance with an embodiment of the invention, amethod for delivering optimal RF power to the plasma load in a plasmachamber while the plasma chamber is provided with a pulsing base RFsignal and at least one non-base RF signal. In step 902, the plasmachamber is powered with a pulsing base RF signal and at least onenon-base RF signal. In step 904, the non-base RF generator(s) is/areoperated in the non-autotuning mode. In step 906, the frequencyswitching of the non-base RF signal is proactively synchronized with thepulsing of the base RF generator. This proactive synchronization enablesthe non-base RF generator to switch its tuned frequency betweenpreviously learned optimal tuned frequency f1 and previously learnedoptimal tuned frequency f2 as the base RF signal pulses between its highstate and its low state. The switching of the tuned frequency of thenon-base RF generator is said to be proactive during production sincethe switching is performed in response to a synchronization signal andis independent of changed plasma load condition sensing (i.e., detectionof changes in chamber parameter/parameters that reflects/reflect suchchanged plasma load condition due to the pulsing of the base RF signal).

The synchronization signal may be issued by the base RF generator, byany RF generator of the multiple RF generators, or by an externalsynchronization circuit or computer, for example. In an embodiment, thenon-base RF generator proactively switches from one previously learnedoptimal RF frequency f1 to another previously learned optimal RFfrequency f2 at the same time as the base RF signal switches from onestate to another state.

For example, if the previously learned optimal RF frequency f1 for thenon-base RF generator is ascertained to be efficient for the high stateof the base RF signal and the previously learned optimal RF frequency f2for the non-base RF generator is ascertained to be efficient for the lowstate of the base RF signal, the non-base RF generator may switch,responsive to a synchronization signal, to previously learned optimal RFfrequency f1 as the base RF generator pulses to the high state. Further,the non-base RF generator may switch, responsive to a synchronizationsignal, to previously learned optimal RF frequency f2 as the base RFgenerator pulses to the low state.

In another embodiment, the non-base RF generator may proactively switchfrom one previously learned optimal RF frequency f1 to anotherpreviously learned optimal RF frequency f2 even slightly before the baseRF signal switches from one state to another state.

In another embodiment, the non-base RF generator may proactively switchfrom one previously learned optimal RF frequency f1 to anotherpreviously learned optimal RF frequency f2 even slightly after the baseRF signal switches from one state to another state.

In one or more embodiments, the proactive non-base RF signal frequencyswitching (e.g., between previously learned RF frequency f1 andpreviously learned RF frequency f2) of the non-base RF generator may becombined with the proactive power level setting of the non-base RFgenerator in order to improve RF power delivery efficiency and stabilityas the base RF signal pulses. In one or more embodiments, the proactivefrequency switching of the non-base RF signal and/or the proactive powerlevel switching of the non-base RF generator may be synchronized withthe pulsing of the base RF signal. If multiple non-base RF signals areinvolved, the frequencies and/or power levels of these non-base RFsignals may be proactively switched as the base RF signal pulses using asimilar arrangement as that discussed for the single non-base RF signalsituation.

As can be appreciated from the foregoing, embodiments of the inventionimprove RF power delivery stability and efficiency as the base RF signalpulses between its high state and its low state. By proactively changingthe RF power levels of the non-base RF generator or non-base RFgenerators (if multiple non-base RF generators are involved), powerdelivery stability is improved while the base RF signal pulses betweenits high state and its low state. By proactively switching betweenpreviously learned optimal RF frequencies for the non-base RF generatoror non-base RF generators (if multiple non-base RF generators areinvolved), power delivery efficiency is improved or power delivery isrendered possible for every pulse state of the base RF signal and forthe time durations surrounding the transition from high-to-low andlow-to-high of the base RF

By providing these additional control knobs, the process recipe windowmay be opened to accommodate more stringent process requirements,leading improved processing and yield of high density/high performancedevices.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andapparatuses of the present invention. Although various examples areprovided herein, it is intended that these examples be illustrative andnot limiting with respect to the invention.

What is claimed is:
 1. A system comprising: a base radio frequency (RF)generator configured to operate at a first frequency; a non-base RFgenerator configured to operate at a second frequency; an externalcontroller coupled to the base RF generator and the non-base RFgenerator via a connection, wherein the external controller isconfigured to: generate a control signal; send the control signal to thebase RF generator operating at the first frequency, wherein the controlsignal is sent via the connection to control pulsing of a base RF signalhaving the first frequency, wherein the base RF signal is pulsed betweena first base power level and a second base power level, wherein the baseRF signal is supplied to a single impedance matching network coupled toa lower electrode of a plasma chamber, wherein the supply of the base RFsignal creates a disturbance in an impedance of plasma within the plasmachamber; and send the same control signal to the non-base RF generatoroperating at the second frequency, wherein the same control signal issent to the non-base RF generator to control pulsing of a non-base RFsignal having the second frequency, wherein the non-base RF signal ispulsed between a first pre-determined non-base power level and a secondpre-determined non-base power level in accordance with the same controlsignal to achieve synchronization with the pulsing of the base RFsignal, wherein the non-base RF signal is supplied to the singleimpedance matching network coupled to the lower electrode, wherein thesynchrinization occurs when the non-base RF signal achieves the firstpre-determined non-base power level simultaneously with achievement ofthe first base power level and achieves the second pre-determinednon-base power level simultaneously with achievement of the second basepower level, wherein the synchronization between the base RF signal andthe non-base RF signal occurs without detection of a change in theimpedance of plasma, wherein the synchronization occurs to reduce thedisturbance in the impedance of plasma caused by the pulsing of the baseRF signal.
 2. The system of claim 1, wherein the base RF generator is a2 megahertz (MHz) RF generator and the non-base RF generator is a 27 MHzor a 60 MHz RF generator.
 3. The system of claim 1, wherein the firstbase power level includes a range of power values of the base RF signaland the range of power values is exclusive of a range of power values ofthe second base power level of the base RF signal.
 4. The system ofclaim 1, wherein the first pre-determined non-base power levelfacilitates achievement of a process result and the secondpre-determined non-base power level facilitates achievement of anotherprocess result.
 5. The system of claim 1, wherein the same controlsignal is sent to the non-base RF generator to achieve the secondpre-determined non-base power level without identifying the change inthe impedance of plasma, wherein the change in the impedance of plasmais caused by a transition from the first base power level to the secondbase power level, and wherein the same control signal is sent to thenon-base RF generator to achieve the wherein the same control signal issent to the non-base RF generator to achieve the first pre-determinednon-base power level without identifying another change in the impedanceof plasma, wherein the other change in the impedance of plasma is causedby a transition from the second base power level to the first base powerlevel.
 6. The system of claim 1, further comprising a computerconfigured to calculate the first pre-determined non-base power leveland the second pre-determined non-base power level from one or moremeasurements of a plasma processing chamber parameter, wherein the firstpre-determined non-base power level and the second pre-determinednon-base power level are calculated to achieve a process parameterassociated with a substrate within the plasma chamber.
 7. The system ofclaim 6, wherein the plasma processing chamber parameter is a chuck biasand the process parameter is a deposition rate.
 8. The system of claim1, further comprising a computer configured to calculate the firstpre-determined non-base power level and the second pre-determinednon-base power level before the base RF generator is controlled togenerate the base RF signal and before the non-base RF generator issynchronized with the base RF generator to generate the non-base RFsignal.
 9. The system of claim 1, wherein both the base RF generator andthe non-base RF generator are configured to be coupled to an electrodeof the plasma chamber via the single impedance matching network, whereinanother electrode of the plasma chamber is configured to be coupled to aground potential, wherein the other electrode is configured to face theelectrode.
 10. The system of claim 1, wherein the achievement of thefirst pre-determined non-base power level and the achievement of thesecond pre-determined non-base power level occurs during processing of asubstrate in the plasma chamber, wherein the change in the impedance ofplasma is not detected during the processing of the substrate.
 11. Asystem comprising: a base radio frequency (RF) generator configured tooperate at a first frequency; a non-base RF generator configured tooperate at a second frequency; an external controller coupled to thebase RF generator and the non-base RF generator via a connection,wherein the external controller is configured to: generate a controlsignal; send the control signal to the base RF generator operating atthe first frequency, wherein the control signal is sent via theconnection to control pulsing of a base RF signal having the firstfrequency, wherein the base RF signal is pulsed between a first basepower level and a second base power level, wherein the base RF signal issupplied to a single impedance matching network coupled to a lowerelectrode of a plasma chamber, wherein the supply of the base RF signalcreates a disturbance in an impedance of plasma within the plasmachamber; and send the same control signal to the non-base RF generatoroperating at the second frequency, wherein the same control signal issent to the non-base RF generator to control pulsing of a non-base RFsignal having the second frequency, wherein the non-base RF signal ispulsed between a first pre-determined non-base frequency and a secondpre-determined non-base frequency in accordance with the same controlsignal to achieve synchronization with the pulsing of the base RFsignal, wherein the non-base RF signal is supplied to the singleimpedance matching network coupled to the lower electrode, wherein thesynchronization occurs when the non-base RF signal achieves the firstpre-determined non-base frequency simultaneously with achievement of thefirst base power level and achieves the second pre-determined non-basefrequency simultaneously with achievement of the second base powerlevel, wherein the synchronization between the base RF signal and thenon-base RF signal occurs without detection of a change in the impedanceof plasma.
 12. The system of claim 11, wherein the first base powerlevel includes a range of power values of the base RF signal and therange of power values is exclusive of a range of power values of thesecond base power level of the base RF signal.
 13. The system of claim11, wherein the same control signal is sent to the non-base RF generatorto achieve the second pre-determined non-base frequency withoutidentifying the change in the impedance, wherein the change in theplasma impedance is caused by a transition from the first base powerlevel to the second base power level and wherein the same control signalis sent to the non-base RF generator to achieve the wherein the samecontrol signal is sent to the non-base RF generator to achieve the firstpre-determined non-base frequency without identifying another change inthe impedance, wherein the other change in the impedance is caused by atransition from the second base power level to the first base powerlevel.
 14. The system of claim 11, further comprising: a computerconfigured to calculate the first pre-determined non-base frequency andthe second pre-determined non-base frequency to increase efficiency ofpower delivered by the non-base RF generator.
 15. The system of claim11, further comprising: a computer configured to calculate the firstpre-determined non-base frequency and the second pre-determined non-basefrequency before the base RF generator is controlled to generate thebase RF signal and before the non-base RF generator is synchronized togenerate the non-base RF signal.
 16. The system of claim 11, wherein thebase RF generator and the non-base RF generator are configured to becoupled to an electrode of the plasma chamber via the single impedancematching network, wherein the plasma chamber includes another electrodethat is configured to be coupled to a ground potential, wherein theother electrode is configured to face the electrode.
 17. The system ofclaim 11, wherein the achievement of the first pre-determined non-basefrequency and the second pre-determined non-base frequency occurs duringprocessing of a substrate in the plasma chamber, wherein the change inthe impedance is not detected during the processing of the substrate.18. A system comprising: a plasma chamber configured to process asubstrate, wherein the plasma chamber includes a lower electrode: asingle impedance matching network coupled to the lower electrode of theplasma chamber; a base radio frequency (RF) generator coupled to thesingle impedance matching network and coupled to an external controllervia a connection, wherein the base RF generator is configured to receivea control signal via the connection from the external controller andoperate at a first frequency to generate a base RF signal that pulses insynchronization with the control signal, wherein the base RF generatoris configured to supply the base RF signal to the single impedancematching network, wherein the supply of the base RF signal creates adisturbance in an impedance of plasma within the plasma chamber; and anon-base RF generator coupled to the single impedance matching networkand coupled to the external controller via the connection, wherein thenon-base RF generator is configured to receive the same control signalfrom the external controller and operate at a second frequency togenerate a non-base RF signal that pulses between a first pre-determinedparameter level and a second pre-determined parameter level insynchronization with the same control signal, wherein thesynchronization of the non-base RF signal with the same control signaloccurs without detection of a change in the impedance of plasma.
 19. Thesystem of claim 18, wherein the first pre-determined parameter level isa power level and the second pre-determined parameter level is a powerlevel.
 20. The system of claim 18, wherein the first pre-determinedparameter level is a frequency and the second pre-determined parameterlevel is a frequency.
 21. The system of claim 18, wherein the secondpre-determined parameter level is achieved without detecting the changein the impedance of plasma in the plasma chamber caused by a transitionfrom a first base power level of the base RF signal to a second basepower level of the base RF signal, and the first pre-determinedparameter level is achieved without detecting a change in the impedanceof plasma in the plasma chamber caused by a transition from the secondbase power level to the first base power level.
 22. The system of claim1, wherein the first and second pre-determined non-base power levels areindependent from the first and second base power levels that are appliedto process a substrate.
 23. The system of claim 1, wherein the singleimpedance matching network matches an impedance of the base RF generatorand the non-base RF generator with an impedance of the plasma chamber.24. The system of claim 1, wherein the same control signal is generatedbased on timing of pulsing of an RF signal output from the non-base RFgenerator.